A Syrian Army source gives the first account of what is believed to have been a chemical attack – and it could mean that one of the West’s biggest fears is about to come true. Channel 4’s Alex Thomson reports.

Black smoke rise from buildings due to government forces shelling, in Aleppo Photo: AP/AMC

By Alex Thomson, Chief Correspondent, Channel 4 News

6:18PM GMT 23 Mar 2013

Whatever happened last week in the town of Khan al-Assal, west of Aleppo, it achieved something extraordinary in the Syrian civil war: unity among Washington, Moscow and Damascus.

All welcomed the rapid decision by Ban Ki-moon, the United Nations secretary-general, to investigate an alleged chemical attack that reportedly killed 26, including Syrian soldiers.

Unusually, the request for that investigation came from the Syrian regime, which claimed that Islamic jihadist rebels launched a chemical weapons attack. Since then, precious little evidence in any way has come from the area despite an awful lot of diplomatic noise around the world.

However a senior source close to the Syrian Army has given Channel 4 News the first clear account of what he claims is believed to have occurred on Tuesday. He is a trusted and hitherto reliable source who does not wish to be identified.

The Syrian military is said to believe that a home-made locally-manufactured rocket was fired, containing a form of chlorine known as CL17, easily available as a swimming pool cleaner. They claim that the warhead contained a quantity of the gas, dissolved in saline solution.

The source said that the town of Khan al-Assal has been in government control since March 13 but – like so much of the area – has been much fought over and parts of the area change hands with relative frequency. Rebel Sunni groups with al-Qaeda sympathies have been attacking the town, where the population is predominantly Shia.

The military’s version of events is that the home-made rocket was fired at a military checkpoint situated at the entrance to the town. The immediate effects were to induce vomiting, fainting , suffocation and seizures among those in the immediate area.

A second source – a medic at the local civilian hospital – said that he personally witnessed Syrian army helping those wounded and dealing with fatalities at the scene. That Syrian soldiers were among the reported 26 deaths has not been disputed by either side.

The military source who spoke to Channel 4 News confirmed that artillery reports from the Syrian Army suggest a small rocket was fired from the vicinity of Al-Bab, a district close to Aleppo that is controlled by Jabhat al-Nusra – a jihadist group said to be linked with al-Qaeda and deemed a “terrorist organisation” by the US.